OS X: Over time, your battery's lifespan decreases, eventually reaching the point where it won't hold a charge nearly as long as when you bought it. Free app coconutBattery gives you detailed info on your battery's health over time.

A lot of this information you can get from OS X already, but coconutBattery puts it into a really simple, readable window: you can see your current charge, the current maximum capacity of your battery, the original capacity of your battery when you bought it, and more. You can even pull out a panel and save your data at any given point, so that over time you can see how much your battery's health degenerates—and take the necessary steps to keep it lasting as long as possible.